If you are using an active crossover as I would assume so the L/R speakers look like a "large" speaker to the AVR/Oppo then LFE info is sent to them. At least that is the way it is supposed to work, and has worked with all the AVRs and pre/pros I have used to date, but I seem to recall there is an issue with Oppo's bass management so I am not sure. I would check with Oppo or look in the Oppo thread. If you are planning to move to an AVR or pre/pro then it will properly handle bass management.
Until fairly recently (last year or so) I ran stereo subs with an active crossover and separate amps so my L/R Maggies were Large to the AVR, obviating some bass management issues, but meaning I was more limited in placement and tuning the system for room compensation was more difficult. I am currently running my subs as a mono swarm, using the phase controls to get everything in synch, and it has worked well for me.
OB's (and IB's, though you can put a housing behind the IB drivers) tend to not reach as deep and distort more than sealed or ported designs, partly because they are (again "usually") much less efficient. There are always exceptions, natch. IME they work well to fill in the lower octave or so where the Maggies are rolling off, but you need a goodly stack with special drivers to handle the really deep bass. I toyed with more conventional OB designs, don't think I ever set up a W-frame version like that. I eventually got tired of messing with them and designed a servo sub similar to Rythmik (but back in the early 1980's). If I had room, money, and patience I'd set up a stack of OB's beside each of my main Magnepans with regular subs filling in below that.
I tried adding damping attached to the rear of my Maggies ages ago but honestly do not recall the result. I think it worked for the high frequencies well enough, which is where you'll hear the most comb filtering. Won't really do much for bass, but it takes a lot of absorption or really big diffusors for bass anyway. Try it and see, and let us know!
HTH - Don
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley